1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly, it relates to an electrophotographic photosensitive member comprising a low-molecular organic photoconductive material that can impart improved electrophotographic performances.
2. Related Background Art
Organic photosensitive members comprising a photosensitive layer mainly composed of an organic photoconductive compound have a number of advantages compared with inorganic photosensitive members, and a great number of proposals have been made thereon and have been put into practical use.
An electrophotographic photosensitive member comprising a photosensitive layer mainly composed of a photoconductive polymer as typified by poly-N-vinyl carbazole and a charge-moving complex formed of the photoconductive polymer and a Lewis acid such as 2,4,7-trinitro-9-fluorenone has been proposed as the organic photosensitive member of the type mentioned above. Compared with inorganic photoconductive materials, organic photoconductive polymers of such a type are advantageous in lightness in weight film-forming properties and so forth. They, however, are inferior to the inorganic photoconductive materials in respect of sensitivity, durability, and stability against environmental changes, and hence can not always be satisfactory.
On the other hand, function-separated electrophotographic photosensitive members, having the charge-generating function and charge-transporting function respectively assigned to separate substances, have remarkably improved the sensitivity and durability in which the conventional organic photosensitive members have been considered defective. Such function-separated photosensitive members have the advantages that the materials for the respective charge-generating material and charge-transporting material can be selected in a vast range and an electrophotographic photosensitive member having any desired performance can be prepared with relative ease.
Various azo pigments, polycyclic quinone pigments, cyanine coloring matters, squaric acid dyes, pyrylium salts coloring matters, etc. are known as the charge-generating material. Of these, the azo pigments have a strong light-resistance, a great charge-generating power and can be readily synthesized. In view of these, those having various structures have been proposed.
On the other hand, known charge-transporting material are, for example, pyrazoline compounds as disclosed in Japanese Patent Publication No. 52-4188, hydrozone compounds as disclosed in Japanese Patent Publication No. 55-42380 and Japanese Patent Application Laid-Open No. 55-52063, triphenylamine compounds as disclosed in Japanese Patent Application Laid-Open No. 57-195254 and Japanese Patent Publication No. 58-32372, fluorene compounds as disclosed in Japanese Patent Application Laid-Open No. 50-22437 and No. 62-208054, and stilbene compounds as disclosed in Japanese Patent Application Laid-Open No. 54-151955 and No. 58-198043. What are required for these charge-transporting materials are that (i) they ere stable to light and heat, (ii) they are stable to ozone, NO.sub.x, nitric acid, etc. that are generated as a result of corona discharge, (iii) they have a high charge-transporting power. (iv) they have a high compatibility with organic solvents and binders, and (v) they can be readily prepared and are inexpensive. However, although the charge-transporting materials comprising the conventional low-molecular organic compounds can satisfy some of the above requirements, none of them can satisfy all of the requirements on a high level. Thus, there remains room for further improvements.